The Synthesis And High-pressure Studies Of Rare-earth Nitrides And Rare-earth Doped Ain-based Diluted Magnetic Semiconductor Nanostructures

Dilute magnetic semiconductor (DMS) is a new kind of semiconductor material,which refers to the cations of a semiconductor partly and disordered replaced bymagnetic transition metals (TMs), such as Cr, Mn, Fe, Co, and Ni. It combines of bothmagnetic and semiconductor properties and is a good source of spin polarizationcarriers, which can be used in the manufacture of a new generation of spintronicsdevices. Right now, it has become an international research hotspot in the field ofmagneto-electronics.Nanomaterials have small grains, great surface area and large amount of grainboundary atoms up to15-50%. The percentage of the disordered arrangement atomsin the grain surface is far greater than the surface atomic percentage of the crystallinematerials. Nanomaterials have three common structural features such as nanoscalestructural units, large numbers of interface or free surface, various interactionsbetween nanostructure units, which make them possessing many special basicproperties that are superior to traditional solids. The magnetic structures betweennanomaterials and conventional polycrystalline as well as amorphous materials arevery different. For example, the unique crystal structure, magnetic structure andmagnetization characteristics of nanomaterials enable the magnetism completelydifferent from the conventional materials. In situ high-pressure X-ray diffractiontechnology is attracting tremendous attention for its ability to explore thecharacteristic properties and to investigate the phase transition mechanism ofnanomaterials under extreme conditions. Using high-pressure experimentaltechnology to study the high-pressure phase transition behavior and the compressionbehavior of the nanometer materials have become a new and challenging topic. In this paper, we focus on the synthesis and properties studies of rare-earth (Sc、Y) doped AlN nanomaterials. Several factors such as doping concentration andmorphologies, which would affect the magnetic properties have been investigated.Meanwhile, in situ X-ray diffraction studies were carried out using angle dispersivesynchrotron radiation technique to investigate the high-pressure behaviors of thedoped AlN nanomaterials. In addition, we synthesized ScN and YN crystals usingdirect current arc discharge method and investigated the structural and elasticproperties of these nitrides under high pressure by means of in situ X-ray diffraction.More details are as follows：1. We synthesized AlN:Sc nanowires using AlSc alloy as the raw materials underN2atmosphere in a direct current arc discharge equipment. The surface of thenanowires is smooth with approximately50-100nm in diameter and dozens ofmicrometers in long. The crystal structure, morphologies, doping concentration,microstructure, luminescent and magnetic properties of the AlN:Sc nanowires havebeen investigated. We revealed the origin of the photoluminescence and proved thatthe AlN:Sc nanowires are ferromagnetism at room temperature. In considering of theexisting theory and experimental results, the observed ferromagnetism in AlN:Scnanowires should origin from the p orbital interactions of the four N atoms around theAl vacancies. The doping Sc has significantly reduced the formation energy of Alvacancies. Further investigations on the AlN:Sc nanowires with different Scconcentration revealed that, the magnetism improved with the increase in the Scconcentration and the doping closed to saturation when the Sc concentration is3.73%.Further increasing the Sc concentration will not improve the magnetism and willproduce the ScN impurity. Our first-principles calculations have proved that withincreasing the Sc concentration in AlN, the formation energy of Al vacancies reducedand then resulting in high cations vacancy concentration. Magnetic weakening did nothappen throughout the completely doping processes and we attributed this to therestriction of our synthesizing method. We deduced that the Sc substituteconcentration in AlN lattice is low that the distance between the impurity ions is toofar to cause direct antiferromagnetic exchange interaction. 2. We synthesized AlN:Sc hierarchical nanostructures such as unilateral andbilateral nanocombs,6-fold-symmetrical pine structure through the control of gaspressure for the investigations of morphologies effect on the magnetic properties ofAlN:Sc nanostructures. Magnetic analysis revealed that they are ferromagnetism atroom temperature. We found that, the unilateral and bilateral nanocombs didn’texhibit enhanced magnetic properties compared to AlN:Sc nanowires, however, theAlN:Sc6-fold-symmetrical pine structure showed significantly improved magneticproperties which would attributed to its complicated pine structure with increasedsurface defects.3. AlN:Y porous nanostructures such as nanoparticles, nanosheet hierarchicalstructure and quasi-array microtubes have been synthesized using AlY alloy as theraw materials under N2atmosphere in a direct current arc discharge equipment. Thecrystal structure, morphologies, dimensions and microstructures of these structureshave been investigated and the formation mechanism of the quasi-array microtubeshas been discussed. The macroporous properties of the obtained AlN:Y porousnanostructures have been confirmed by N2adsorption-desorption measurementsunder77K. The specific surface area of the obtained AlN:Y nanoparticles, AlN:Ynanosheet hierarchical structure and AlN:Y quasi-array microtubes samples aredetermined to be (17.61±0.05,25.85±0.09and6.73±0.08) m2g1, by using thestandard procedure of the Brunauer-Emmett-Teller gas adsorption method. Wefocused on the H adsorption-desorption properties of AlN:Y nanosheet hierarchicalstructure which exhibits the biggest specific surface area. However, the results turnedout to be disappointed; we deduced that the low amount of unsaturated Al ions in thenanostructures would not afford enough H for our observation.4. In situ X-ray diffraction studies of rare-earth metals Sc-and Y-doped AlNnanoprisms and nanowires were carried out using angle dispersive synchrotronradiation technique in a diamond anvil cell. Pressure induced wurtzite-to-rocksaltphase transitions start at18.6and16.2GPa for the AlN:Sc and AlN:Y nanoprismsand20.3and18.5GPa for the AlN:Sc and AlN:Y nanowires, both of these foursamples showing lower phase-transition pressures compared with bulk AlN and AlN nanowires while slightly higher than that of AlN nanocrystals. Besides, the phasetransition pressures of AlN:Y nanostructures are lower than that of AlN:Scnanostructures in both of the four samples and the doped nanowires exhibit higherphase transition pressures than that of the doped nanoprisms. The effects of volumeexpansion, volume collapse, and crystal defects on phase transition have beendiscussed. The distinct high-pressure behaviors can be explained in terms of thedoping induced Al vacancy defects along with substitute ions sit at the cations sites,which lead to the distortion of the crystal structure that reduce the structural stabilityof the doped AlN nanostructures. Comparing the ionic radii between Sc3+(rSc=0.73) and Y3+(rY=0.89), we deduce that the Y3+would induce a larger distortion inthe AlN:Y crystal structures than that of in AlN:Sc that reduce the phase-transitionpressure. The higher phase transition pressures in the doped AlN nanowires shouldbe attributed to their unique intrinsic geometry of nanowires. The nanowires havelarge aspect ratios with an average size of50-100nm in diameter and dozens ofmicrometers in axial lengths. In addition, the doped AlN nanowires have goodtoughness and the larger bulk modulus in the doped AlN nanowires indicates thathigher pressures are needed to overcome the higher-energy hindrance in the dopedAlN nanowires than in the doped AlN nanoprisms.5. ScN and YN crystals were synthesized through direct nitridation of Sc and Ymetals with nitrogen using plasma assisted direct current arc discharge method.Structural and elemental characterization indicate that the as-synthesized ScNcrystals are stoichiometric single crystalline and YN crystals are nonstoichiometricpolycrystalline with grain sizes range from5-15nm. In consideration of the XRD,EDS and HRTEM results of YN crystals, we deduce the formation of non-crystallinephase of metal Y in YN and this contributes to the high contents of Y in the EDSresults. Investigation on the conditions of formation of rare-earth nitrides has shownthat the formation of yttrium-group metals nitrides in both molecular nitrogen andammonia the limiting value of nitrogen content is not attained (the actual content is8-10%lower than the theoretical). This is attributable to the high dissociationpressure of yttrium-group metal nitrides of limiting compositions. Besides, in the DC arc plasma system the growth of products always experiences high quench rate (i.e.,103K/S) as a result of the large temperature gradient, which would result in thedisordered arrangements of the clusters of Y atoms and then forming non-crystallinestructure upon quenching. High pressure structural and elastic properties of ScN andYN crystals were carried out using angle dispersive synchrotron radiation in adiamond anvil cell up to53.9and53.5GPa, respectively. No phase transitionoccurred in the pressure ranges we achieved in this study, which is in accord with thetheoretical studies that ScN and YN are stable under high pressure. The measuredzero-pressure bulk modulus for ScN coincides with those of theoretical results whileYN yield a bulk modulus much higher than the theoretical values exhibiting reducedcompressibility. It is considered to be caused by the decreased grain sizes in thecharacteristic polycrystalline YN.